IMHO totally depends on what you are doing and who your users are. Generally going for graceful degradation should keep a lot of stuff usable for outdated browsers, even if not full fidelity/pretty, and that often should be enough. If you have lots of IE11 users for whatever reason (i.e. I believe in some countries rates are a lot higher) it might be worth the effort to make it better for them. If you ship an internal app for a company that rolls out a newer standard browser everywhere, target that.